Description: During the Late Pleistocene–Holocene, the Ross Sea Ice Shelf exhibited strong spatial variability in relation to the atmospheric and oceanographic climatic variations. Despite being thoroughly investigated, the timing of the ice sheet retreat from the outer continental shelf since the Last Glacial Maximum (LGM) still remains controversial, mainly due to a lack of sediment cores with a robust chronostratigraphy. For this reason, the recent recovery of sediments containing a continuous occurrence of calcareous foraminifera provides the important opportunity to create a reliable age model and document the early deglacial phase in particular. Here we present a multiproxy study from a sediment core collected at the Hallett Ridge (1800m of depth), where significant occurrences of calcareous planktonic and benthic foraminifera allow us to document the first evidence of the deglaciation after the LGM at about 20.2 ka. Our results suggest that the co-occurrence of large Neogloboquadrina pachyderma tests and abundant juvenile forms reflects the beginning of open-water conditions and coverage of seasonal sea ice. Our multiproxy approach based on diatoms, silicoflagellates, carbon and oxygen stable isotopes on N. pachyderma, sediment texture, and geochemistry indicates that abrupt warming occurred at approximately 17.8 ka, followed by a period of increasing biological productivity. During the Holocene, the exclusive dominance of agglutinated benthic foraminifera suggests that dissolution was the main controlling factor on calcareous test accumulation and preservation. Diatoms and silicoflagellates show that ocean conditions were variable during the middle Holocene and the beginning of the Neoglacial period at around 4 ka. In the Neoglacial, an increase in sand content testifies to a strengthening of bottom-water currents, supported by an increase in the abundance of the tycopelagic fossil diatom Paralia sulcata transported from the coastal regions, while an increase in ice-rafted debris suggests more glacial transport by icebergs.

Description: Downcore sediment grain-size records of mineral dust (2–10 μm) can provide key insights into changes in wind strength and source-area characteristics over glacial-interglacial timescales. However, so far, little is known about glacial-interglacial changes of dust grain size in the open Southern Ocean, which are potentially associated with changes in the strength and position of the southern westerly winds. Here, we analyzed the grain-size distributions of subantarctic deep-sea sediments from the Pacific (PS75/056–1) and Atlantic (ODP Site 1090) sectors of the Southern Ocean, downwind of the major Southern Hemisphere dust source regions. Dust mean grain sizes show opposite trends in the two Southern Ocean sectors. Larger glacial grain sizes are observed in the Pacific sector, while finer glacial grain sizes are observed in the Atlantic sector. In the South Pacific, larger mean dust grain sizes parallel higher Fe fluxes during glacials. In contrast, in the South Atlantic record increased glacial Fe fluxes coincide with a decrease in glacial mean dust grain sizes consistent with some Antarctic ice core records. Our results suggest that the opposing grain-size trends are the result of different responses to glacial conditions in the sources and of changing wind and transport patterns. For the South Pacific, a possible explanation of our results could be an intensification of wind strength over Australia enabling emission of larger dust particles. This strengthening would imply a northward shift of the westerlies which facilitated the transport of dust from enhanced and/or more Australian and New Zealand sources. For the Atlantic, the decreased glacial dust grain size could be the consequence of increased glacial activity in the Patagonian Andes, generating and supplying more and finer-grained dust from the exposed continental shelf to the South Atlantic. These findings indicate that more extensive studies of wind-blown sediment properties in the Southern Ocean can provide important insights on the timing and latitudinal extent of climatic changes in the sources and variations of transport to the Southern Ocean by the westerly winds.

Description: Southern Ocean westerly wind intensity and position are thought to play a crucial role in controlling glacial/interglacial CO2 changes through their impact on Antarctic upwelling intensity and the delivery of iron-rich dust that stimulates biological production during glacial periods. Sediment-core grain size records can provide key insights into changes in wind strength and source-area characteristics over glacial-interglacial timescales. However, so far, little is known about G/IG grain size changes in Southern Ocean sediments. For this study, we analyzed the grain-size distributions of two subantarctic deep sea sediments cores from the Pacific (PS75/056-1) and Atlantic (ODP Site 1090) sectors of the Southern Ocean. Dust mean grain size shows opposing trends in the two Southern Ocean sectors. Coarser glacial grain sizes are observed in the Pacific sector, while finer glacial grain-sizes are observed in the Atlantic. Our results suggest that changes in the latitudinal position of the SWW had distinct impacts on grain size distribution in the Atlantic and Pacific sectors, also likely associated with shifts in the dust source areas. These findings indicate that more extensive studies of grain-size distribution in the Southern Ocean can provide important insights on the timing and latitudinal extent of the westerly winds changes during ice ages.

Description: Marine and terrestrial geological and marine geophysical data that constrain deglaciation since the Last Glacial Maximum (LGM) of the sector of the West Antarctic Ice Sheet (WAIS) draining into the Amundsen Sea and Bellingshausen Sea have been collated and used as the basis for a set of time-slice reconstructions. The drainage basins in these sectors constitute a little more than one-quarter of the area of the WAIS, but account for about one-third of its surface accumulation. Their mass balance is becoming increasingly negative, and therefore they account for an even larger fraction of current WAIS discharge. If all of the ice in these sectors of the WAIS was discharged to the ocean, global sea level would rise by ca. 2 m. There is compelling evidence that grounding lines of palaeo-ice streams were at, or close to, the continental shelf edge along the Amundsen Sea and Bellingshausen Sea margins during the last glacial period. However, the few cosmogenic surface exposure ages and ice core data available from the interior of West Antarctica indicate that ice surface elevations there have changed little since the LGM. In the few areas from which cosmogenic surface exposure ages have been determined near the margin of the ice sheet, they generally suggest that there has been a gradual decrease in ice surface elevation since pre-Holocene times. Radiocarbon dates from glacimarine and the earliest seasonally open marine sediments in continental shelf cores that have been interpreted as providing approximate ages for post-LGM grounding-line retreat indicate different trajectories of palaeo-ice stream recession in the Amundsen Sea and Bellingshausen Sea embayments. The areas were probably subject to similar oceanic, atmospheric and eustatic forcing, in which case the differences are probably largely a consequence of how topographic and geological factors have affected ice flow, and of topographic influences on snow accumulation and warm water inflow across the continental shelf. Pauses in ice retreat are recorded where there are “bottle necks” in cross-shelf troughs in both embayments. The highest retreat rates presently constrained by radiocarbon dates from sediment cores are found where the grounding line retreated across deep basins on the inner shelf in the Amundsen Sea, which is consistent with the marine ice-sheet instability hypothesis. Deglacial ages from the Amundsen Sea Embayment (ASE) and Eltanin Bay (southern Bellingshausen Sea) indicate that the ice sheet had already retreated close to its modern limits by early Holocene time, which suggests that the rapid ice thinning, flow acceleration, and grounding line retreat observed in this sector over recent decades are unusual in the context of the past 10,000 years.

Description: Recent palaeoglaciological studies on the West Antarctic shelf have mainly focused on the wide embayments of the Ross and Amundsen seas in order to reconstruct the extent and subsequent retreat of the West Antarctic Ice Sheet (WAIS) since the Last Glacial Maximum (LGM). However, the narrower shelf sectors between these two major embayments have remained largely unstudied in previous geological investigations despite them covering extensive areas of the West Antarctic shelf. Here, we present the first systematic marine geological and geophysical survey of a shelf sector offshore from the Hobbs Coast. It is dominated by a large grounding zone wedge (GZW), which fills the base of a palaeo-ice stream trough on the inner shelf and marks a phase of stabilization of the grounding line during general WAIS retreat following the last maximum ice-sheet extent in this particular area (referred to as the Local Last Glacial Maximum, ‘LLGM’). Reliable age determination on calcareous microfossils from the infill of a subglacial meltwater channel eroded into the GZW reveals that grounded ice had retreated landward of the GZW before ∼20.88 cal. ka BP, with deglaciation of the innermost shelf occurring prior to ∼12.97 cal. ka BP. Geophysical sub-bottom information from the inner-, mid- and outer shelf indicates grounded ice extended to the shelf edge prior to the formation of the GZW. Assuming the wedge was deposited during deglaciation, we infer the timing of maximum grounded ice extent occurred before ∼20.88 cal. ka BP. This could suggest that the WAIS retreat from the outer shelf was already underway during or even prior to the global LGM (∼23–19 cal. ka BP). Our new findings give insights into the regional deglacial behaviour of this understudied part of the West Antarctic shelf and at the same time support early deglaciation ages recently presented for adjacent drainage sectors of the WAIS. If correct, these findings contrast with the hypothesis that initial deglaciation of Antarctic Ice Sheets occurred synchronously at ∼19 cal. ka BP.

Description: Although the climate development over the Holocene in the Northern Hemisphere is well known, palaeolimnological climate reconstructions reveal spatiotemporal variability in northern Eurasia. Here we present a multi-proxy study from north-eastern Siberia combining sediment geochemistry, and diatom and pollen data from lake-sediment cores covering the last 38,000 cal. years. Our results show major changes in pyrite content and fragilarioid diatom species distributions, indicating prolonged seasonal lake-ice cover between ∼13,500 and ∼8900 cal. years BP and possibly during the 8200 cal. years BP cold event. A pollen-based climate reconstruction generated a mean July temperature of 17.8 °C during the Holocene Thermal Maximum (HTM) between ∼8900 and ∼4500 cal. years BP. Naviculoid diatoms appear in the late Holocene indicating a shortening of the seasonal ice cover that continues today. Our results reveal a strong correlation between the applied terrestrial and aquatic indicators and natural seasonal climate dynamics in the Holocene. Planktonic diatoms show a strong response to changes in the lake ecosystem due to recent climate warming in the Anthropocene. We assess other palaeolimnological studies to infer the spatiotemporal pattern of the HTM and affirm that the timing of its onset, a difference of up to 3000 years from north to south, can be well explained by climatic teleconnections. The westerlies brought cold air to this part of Siberia until the Laurentide ice-sheet vanished 7000 years ago. The apparent delayed ending of the HTM in the central Siberian record can be ascribed to the exceedance of ecological thresholds trailing behind increases in winter temperatures and decreases in contrast in insolation between seasons during the mid to late Holocene as well as lacking differentiation between summer and winter trends in paleolimnological reconstructions.

Description: Inter-ice stream areas cover significant portions of Antarctica's formerly glaciated shelves, but have been largely neglected in past geological studies because of overprinting by iceberg scours. Here, we present results of the first detailed survey of an inter-ice stream ridge from the West Antarctic continental shelf. Well-preserved sub- and proglacial bedforms on the seafloor of the ridge in the eastern Amundsen Sea Embayment (ASE) provide new insights into the flow dynamics of this sector of the West Antarctic Ice Sheet (WAIS) during the Last Glacial cycle. Multibeam swath bathymetry and PARASOUND acoustic sub-bottom profiler data acquired across a mid-shelf bank, between the troughs of the Pine Island-Thwaites (PITPIS) and Cosgrove palaeo-ice streams (COPIS), reveal large-scale ribbed moraines, hill-hole pairs, terminal moraines, and crevasse-squeeze ridges. Together, these features form an assemblage of landforms that is entirely different from that in the adjacent ice-stream troughs, and appears to be unique in the context of previous studies of Antarctic seafloor geomorphology. From this assemblage, the history of ice flow and retreat from the inter-ice stream ridge is reconstructed. The bedforms indicate that ice flow was significantly slower on the inter-ice stream ridge than in the neighbouring troughs. While terminal moraines record at least two re-advances or stillstands of the ice sheet during deglaciation, an extensive field of crevasse-squeeze ridges indicates ice stagnation subsequent to re-advancing ice, which deposited the field of terminal moraines in the NE. The presented data suggest that the ice flow behaviour on the inter-ice stream ridge was substantially different from that in the adjacent troughs. However, newly obtained radiocarbon ages on two sediment cores recovered from the inter-ice stream ridge suggest a similar timing in the deglaciation of both areas. This information closes an important gap in the understanding of past WAIS behaviour in the eastern ASE. Our newly-documented bedforms will also serve as an important diagnostic tool in future studies for interpreting ice-sheet histories in similar inter-ice stream areas.

Description: The diatom oxygen isotope composition (δ18Odiatom) from lacustrine sediments helps tracing the hydrological and climate dynamics in individual lake catchments, and is generally linked to changes in temperature and δ18Olake. Lake Bolshoye Shchuchye (67°53′N; 66°19′ E; 186 m a.s.l) is the largest and deepest freshwater reservoir in the Polar Urals, Arctic Russia. The diatom oxygen isotope interpretation is supported by modern (isotope) hydrology, local bioindicators such as chironomids, isotope mass-balance modelling and a digital elevation model of the catchment. The Bolshoye Shchuchye δ18Odiatom record generally follows a decrease in summer insolation and the northern hemisphere (NH) temperature history. However, it displays exceptional, short-term variations exceeding 5‰, especially in Mid and Late Holocene. This centennial-scale variability occurs roughly contemporaneously with and similar in frequency to Holocene NH glacier advances. However, larger Holocene glacier advances in the Lake Bolshoye Shchuchye catchment are unknown and have not left any significant imprint on the lake sediment record. As Lake Bolshoye Shchuchye is deep and voluminous, about 30–50% of its volume needs to be exchanged with isotopically different water within decades to account for these shifts in the δ18Odiatom record. A plausible source of water with light isotope composition inflow is snow, known to be transported in surplus by snow redistribution from the windward to the leeward side of the Polar Urals. Here, we propose snow melt variability and associated influx changes being the dominant mechanism responsible for the observed short-term changes in the δ18Odiatom record. This is the first time such drastic, centennial-scale hydrological changes in a catchment have been identified in Holocene lacustrine diatom oxygen isotopes, which, for Lake Bolshoye Shchuchye, are interpreted as proxy for palaeo precipitation and, on millennial timescales, for summer temperatures.

Description: Rapid regional warming at an increasing pace ever since the end of the Little Ice Age (c. AD 1900) causes significant change in the coastal marine environments of the West Antarctic Peninsula and beyond. A comprehensive set of hydroacoustic ground-discrimination data (RoxAnn GDX) was gathered to develop a high resolution characterization of the seafloor habitats in the Potter Cove, King George Island, a small fjord with a retreating former tidewater glacier at its head. Sediment samples and underwater video footage are used for ground truthing. Seven habitat zones are distinguished. These include the shallow high-energy wave zone exposing unvegetated rocks to the low-energy deeper basins characterized by muddy sediments and the typical biota including ophiuroids, ascidians, sponges, sea pens. The results allow to subdivide the Potter Cove into a “dynamic zone” (DZ) with rocks and mixed fine sediments covering the inner cove, a large transition zone that we call the “subrecent zone” (SZ) buried under fine meltwater sediments and the “quasi persistent zone” (QPZ) that reveals more mature conditions in many aspects further downfjord. These zones represent development stages resulting from the increasing distance to and decreasing influence of the glacier front. The DZ is trailing directly behind the retreating glacier front. As long as there is strong discharge of sediment-loaded meltwater the DZ transitions into the SZ after a period of time (under recent conditions: decades) which itself transitions into the QPZ after centuries. We assume that during the Medieval Warm Period (c. AD 800-1350) the glacier terminus was at or even behind its present position. Until the maximum of the Little Ice Age the glacier advanced to form a prominent moraine complex. Ever since the glacier retreated at increasing speed to its recent position. If the warming trend continues the glacier will retreat further away from the fjord head and the QPZ will likely cover the entire fjord after several centuries.